[TCML] SISG Pig - day 2

[FIFTYGUY@xxxxxxx]

Folks-
 
    From my last post, I had decided I needed a  different primary cap. But 
after I had spent some time today assembling another  MMC, I realized, "what 
the heck?" So once again I adopted the "bigger hammer"  theory of coil testing...
    I dusted off one of the bigger Maxwells (#31976,  .075 uF at 40kV, 14 
lbs!) and stuck it on the 6" coil. I adjusted primary tap  accordingly. Now that 
made a BIG difference!
    I didn't do any measuring, but sparks were now over  70". Much louder! 
With the six SISG4 sections, the bang size was now a nominal  17.5 Joules. No 
idea how high the BPS was, but it was still plenty fast! Much  faster than 120 
BPS. At full variac, it actually settled down into a good,  clean, modulated 
roar. I could hear the sound echoing off the houses down the  block. 
    All was fine, until a nasty strike arced around  *under* the primary and 
hit something under there (the cap leads, I think). Then  it would only 
sputter, but with full current draw on the pig. A few seconds  later, we saw some 
flames, and realized the bridge rectifier was damaged. In  fact, it had turned 
several of the UF4007's in one of the bridge legs into soot  marks on the 
plastic base. It had somehow actually blown apart the soldered  string into several 
smaller pieces. 
    So I clipped out a half-dozen bad diodes, and  soldered the string back 
together. After this string blew up near the  repair point, I repeated the 
process. This failed as well. Since I couldn't find  my bag of UF4007 diodes, I 
dug out one of the salvaged "bar"  rectifiers from an old 3-phase HV power 
supply that ran a 20kW AM radio station.  I guessed it was rated around an amp at 
around 20kV, so I wired it in place of  the damaged leg. Worked fine the rest 
of the night!
    BTW, the damaged leg was from one AC terminal to  the positive bridge 
terminal. But the cap was on the negative side of the SISG.  Anyhow, I put a big 
piece of sheet steel on the ground next to the cap, and for  the rest of the 
night the arcs hit the steel instead of the cap. 
    Many primary hits, many ground strikes, and a few  secondary 
midpoint-to-strike rail hits. 18 joules per bang, and Lord knows what  BPS for half an 
hour cumulative. And still the SISG works fine, and the IGBT  heatsinks are 
barely warm at all! I haven't played with the coupling yet to see  how tight I can 
get it. I've got enough caps to make it run "resonant" with the  pig at 120 
bps, but I'd probably burn up the 6" coil! On the other hand it  would be a good 
test of how the SISG handles a very low Surge Impedance...
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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